If they kill the crew, there is no ransom. Almost guaranteed military action, and you know Obama is licking his chops to set up some African bases to usurp some Chinese influence in the region.
If the next pirate/USA showdown gets messy (hostages killed, etc), I think they STILL don't keep it up... as they know it's bullets in heads for all their guys, and no $ paid, no matter what.
You have to think that while they're pissed about it, they're not suicidal. When Reuters claled one of the pirates on sat phone, he was scared shitless and asked the reporter to pray for them.